Beam shaping is an important concept in optics, and is commonly involved in a variety of optical components and instruments. Focusing an optical beam through dielectric lenses is a good example, and their operation is well understood on the basis of classical optics, i.e., curved surfaces with an index contrast provide refraction and focusing of light. In terms of wave optics, the optical field emanating from a lens can be expressed as Fourier expansion of radiation from infinitesimal dipoles on an exit surface of the lens. The role of the lens is to provide a phase correction to each of the Fourier components by virtue of phase retardation resulting from path length difference. The optical fields then reassemble to a focus at some distance beyond the lens, forming an image of the dipole source.